This invention relates to ink-jet printing mechanisms, and more particularly, to drive motors used in ink-jet printers, plotters, scanners, facsimile machines, and the like.
An inkjet printing mechanism is a type of non-impact printing device which forms characters and other images by controllably spraying drops of ink from a printhead. Inkjet printing mechanisms may be employed in a variety of devices, such as printers, plotters, scanners, facsimile machines, and the like. For convenience, inkjet printers are used herein to illustrate the concepts of the present invention.
The printhead ejects ink through multiple nozzles in the form of drops which travel across a small air gap and land on a recording media. The drops are very small. Inkjet printers commonly print within a range of 180 to 600 dots per inch (dpi). The ink drops dry on the recording media shortly after deposition to form the desired printed images.
There are various types of inkjet printheads including, for example, thermal inkjet printheads and piezoelectric inkjet printheads. By way of example, for a thermal inkjet printhead, ink droplets are ejected from individual nozzles by localized heating. A small heating element is disposed at individual nozzles. An electrical current is passed through the element to heat it. This causes a tiny volume of ink to be rapidly heated and vaporized by the heating element. The vaporization causes the ink to be ejected through the nozzle. A driver circuit is coupled to individual heating elements to provide the energy pulses and thereby controllably deposit ink drops from associated individual nozzles. Such drivers are responsive to character generators and other control logic to energize selected nozzles of the printhead for forming desired images on the recording media.
During printing, ink tends to build up at the nozzle orifices on the printhead. This build-up can be caused by ink droplets that are not completely ejected, excess ink at the orifice that is not vaporized during ejection, or ink splatterings that reflect from the recording media. The resident ink on the printhead can clog the nozzle orifices and detrimentally disrupt or impair proper printing.
Conventional inkjet printers are often equipped with service station mechanisms that include wiper assemblies designed to periodically clean the nozzle section of the inkjet printhead to remove any resident ink. Typically, the wiper assembly has an individual wiper for each printhead which engages and scrubs the printhead orifices. The wiper assembly is alternately moved to an activated position suitable for cleaning the printhead and then to a retracted position where it does not interfere with the printhead during printing.
A service station can be configured to perform different printhead maintenance tasks. Capping nozzles when they are not in use is another example of a service station function.
A service station must typically be moved between two or more positions. In many configurations, there is a need to control the position and/or velocity of the motor and associated mechanism. This is often accomplished with motor positional feedback provided by an integrated or external encoder. In some environments, such as in magnetic-based hard-disk drives, feedback is provided without an encoder, by using back-EMF sensing. However, this technique is not commonly used in conjunction with brush-type DC motors. Furthermore, back-EMF sensing is used primarily for velocity control rather than position control.
Step motors can be used in some situations to eliminate the cost and complexity associated with external feedback devices. However, step motors are noisy and subject to positional errors.
The inventors have found a way to monitor and control the velocity and position of a conventional, brush-type, DC motor, without requiring an external encoder. When used to position a service station in an inkjet printer, cost is reduced by eliminating the need for stepper motors or encoders.